[FEATURE] Add a `PythonCallback` similar to `CustomJS`

Add a new model to allow a python-side callback that’s called from the browser. If this is possible, I didn’t find any way to do that in the docs. Note that this can also simplify the event-mechanism as there will not be longer be a need for both on_event and js_on_event.

Looking briefly on the code, I have an implementation suggestion (tell me what you think about it): In the python side, it just wraps a callback. On the browser side, this could be implemented by triggering a new type of event on calling CallbackLike<..>.execute, which, in turn, triggers a wrapper in the python-side that calls the given callback.

[bryevdv]

• It’s more complicated, not less. You can just currently define a function and pass to on_event with no imports. To use some PythonCallback you have to: import it (know to import it), define a function and configure it, or subclass is and define a method, then configure that result on the model. That is strictly always more work, not less (and more and more ways to have to document)

• This could only work with a Bokeh server because the Bokeh server is the Python process that will execute python code. (I hope that is understood here but I am not sure). It cannot work at all with standalone output (i.e. output_file + show) but it seems like it would be very easy for users to be confused on this point, which directly translates into an inctreased support burden.

Also, all the tools that have a callback property (there are quite a few),

There are only three left. CustomAction, TapTool, and HoverTool It used to be the case there there were lots but the plan (which has been ongoing) is to get rid of all of the old ad-hoc callback properties and have only the one consistent on_event / js_on_event mechanism everywhere. These will all be deprecated some day.

This is duplicative of existing functionality but will require more work to develop, maintain, document, and explain. We should continue the work to unify everything event-related under common on_event / js_on_event which is easier to explain via parallel comparison. If we need new events, we can add them. But I am a strong -1 on a differentPythonCallback that needs to be maintained (and explained) differently. .

[bryevdv]

FYI I will go ahead and close this with noaction if others feel strongly please make a case to re-open